Fuel ignitability and compression ratio dependence of a premixed charge compression ignition engine

Decreasing the compression ratio improves thermal efficiency in a premixed charge compression ignition engine with direct early injection for the range of investigated octane numbers up to the limit of stable ignition. This is because the improvements in the degree of constant volume combustion and decreased cooling loss as the ignition timing approached TDC Outweighs the reductions in combustion efficiency and theoretical efficiency. The indicated thermal efficiency with different octane number fuels is almost unchanged if the compression ratio is optimised for the fuel. The thermal efficiency benefits of lower compression ratios decrease with the higher homogeneity achieved by advancing the injection timing, due to a corresponding deterioration in combustion efficiency. However, the qualitative dependence of the indicated thermal efficiency and the exhaust g-as emissions on fuel ignitability and the compression ratio are Unaffected by changes in the mixture homogencity. In addition, EGR efficiently suppresses premature ignition and reduces Cooling losses Without deteriorating combustion efficiency.